Mirafiori lettuce virus (MiLV) was isolated from lettuce showing big-vein symptoms in Italy in 2000 (P. Roggero et al., (2000) Archives of Virology 145: 2629-2642). In 2002, MiLV, rather than Lettuce Big-Vein Virus (LBVV), was reported to be the causative virus of Lettuce big-vein disease (H. Lot et al., (2002) Phytopathology 92: 288-293). MiLV is a soil-borne virus transmitted by the filamentous fungus Olpidium brassicae. It has become a problem in the U.S., Japan, and Europe for causing the Lettuce Big-vein disease. The MiLV virus has been reported to be a member of the Ophiovirus genus, comprising three genomic minus-strand RNA segments of 8.5 kb, 1.9 kb, and 1.7 kb, and a 48-kDa coat protein as the structural protein. Since this virus has been discovered only recently and its genetic information and such remain unrevealed, no reliable genetic diagnostic methods have been established.
There are several varieties of lettuce that are resistant to MiLV diseases, but their resistance is weak. Furthermore, no useful resistant genetic material has been found. Thus, methods of introducing viral genes into plants will be useful for producing plants with a high-level resistance to this virus. For this purpose, it is necessary to identify the viral gene sequence.